Medullary canal reamers of this type are used to enlarge the medullary canals of bone in preparation for the insertion of fixation devices, performing an intramedullary osteotomy, stimulating bone growth, the insertion of a plug to preclude bone cement from migrating while it is in its viscous state, and for other reasons. The medullary canals of bone are seldom straight. More typically the canal will have some degree of curvature to it. Should a straight and rigid series of reamers be employed to enlarge the canal there is considerable likelihood that the reamer, in not being capable of following the bone's curvature, will not remove the desired uniform amount of bone tissue. In such a situation, excessive tissue removal occurring in at least one plane will be experienced as the reamer is advanced. For this reason medullary canals are almost always prepared with reamers having a flexible shaft.
Heretofore, flexible medullary reamers are of two types. The first to come into common usage consisted of spiral or helically wound metal wire(s) or strip(s), which comprised the shaft of the reamer. A disadvantage of this design is that these reamers can be operated only in the forward mode of rotation. If operated in the reverse mode, which occasionally is required to free a lodged reamer and to facilitate normal removal, the shaft unwinds, thus rendering the reamer permanently deformed, unusable, and unrepairable. This adds considerably to the cost of maintaining a serviceable set of medullary reamers. Further, a lodged cutting head may subsequently be extremely difficult, if not impossible to remove without further violation of the involved bone and surrounding tissues. Another disadvantage of said design is the extreme difficulty in their proper and thorough cleaning after use. The spiral or helically wound metal shafts contain many voids of various sizes. Blood and tissue readily infiltrate such voids and become trapped within the confines of the shaft. When the reamer is in use the voids are considerably distorted and enlarged as the reamer is advanced towards and within the medullary canal, thus providing ready access for the particles of tissue. Prior to use, all medullary reamers are sterilized and hopefully, the blood and tissue particles not evacuated during the cleaning process and remaining within the interstices of the reamers, are at least rendered harmless. However, depending upon the amount and composition of the extraneous particles and their degree of isolation from the sterilizing process, said particles may not be rendered sterile. Even in a sterile condition these foreign particles may still cause problems of infection should they become dislodged from the confines of the reamer and come into contact with the patient's internal tissues. Medical professionals recognize this problem but acquiesce to using these reamers for lack of an acceptable alternative. A further disadvantage of this medullary reamer is that the torsional load it is subject to when in use results in poor power transfer and varying degrees of distortion of said shaft. If the power source providing the rotational energy to the reamer is great enough, said coils may tighten sufficiently to adversely affect the intended flexibility of the shaft. Another disadvantage associated with a psiral or helically wound reamer is the trauma it imposes to surrounding tissues. This results when the shaft of the reamer is not completely within the medullary canal as would occur during the initial reaming process. As the shaft rotates, that portion remaining outside of the medullary canal can become excessively flexed and distorted, thus enlarging the voids between the coils of the shaft. As the flexed shaft rotates, tissue lying outside of the canal and unintended for removal, becomes trapped within the voids and are torn from their underlying structures.
A second distinct type of flexible medullary reamer is described in the literature. The shaft of the reamer embodies a plurality of parallel, flexible elements joined together at opposite ends by means of a welded or soldered connection. A disadvantage of this shaft is that proper cleaning is difficult to accomplish. Towards its opposite ends where the individual elements converge, said elements come in close contact with each other until the final termination point is reached where they are permanently welded or soldered together to form a solid mass. Where these elements begin to converge, blood and tissue can readily become trapped and prove difficult to remove during the cleaning process. Another disadvantage of this flexible reamer is the excessive noise generated in its use which is caused by the individual elements being twisted and forcefully whipped into contact with each other. A further disadvantage of said flexible reamer occurs during usage. As rotation occurs, the individual elements spirally tighten around each other causing the shaft to become more rigid and thereby reducing the shaft's flexibility and increasing the likelihood of the attached cutting head not properly adhering to the central path of the medullary canal. Another disadvantage of said flexible reamer is the shaft's tendency, as it rotates but not yet fully within the confines of the medullary canal, to tear tissue from underlying structures as the individual elements are torsionally loaded and unloaded, thereby enlarging and contracting the spaces between the individual elements sufficiently to trap uninvolved tissue between the individual wires and tearing them free. Another disadvantage of said flexible reamer occurs in attempting to utilize the central bore of the reamer. The central bore is intended to receive a long small diameter guide pin which had previously been inserted into the medullary canal to act as a track for the advancing reamer. Except at its respective ends, this reamer lacks a well-defined and bordered central bore. Therefore, it is difficult to prevent the guide pin from exiting the reamer in the area of the free standing wires during the initial positioning of the guide pin within the reamer. A further disadvantage of this flexible shaft is the inefficient transfer of energy from the power source to the cutting head which is caused by the twisting and wrapping together of the individual elements as the reamer is rotated.